The present invention relates to a method of producing a shadow mask of a television, and, more particularly, to a method of producing an aperture grill of a thickness of from 20 to 100 .mu.m, for example, used typically in a Trinitron (Trademark of Sony Corporation) cathode ray tube (CRT).
In recent years, CRT display devices such as color televisions have become large in size, so that shadow masks used in such devices are required to be of large size as well. Particularly, in the type of shadow masks called aperture grills having vertical slits, the manner of fixing the same is different from the manner of fixing other types of shadow masks having other types of slits or circular openings. That is, the aperture grill is fixed under tension to a rigid frame. For this reason the frame must necessarily be enlarged in relation to the enlarged aperture grill and is required to resist the tension necessary for the fixing of the aperture grill of the conventional thickness. As a consequence, the weight of the frame is increased remarkably. In order to cope with this increase in the weight of the frame, the weight of the aperture grill must be reduced so that the thickness of the aperture grill will have to be reduced to compensate for the enlargement of the size.
The thickness of conventional shadow masks having slits or circular holes, including aperture grills having vertical slits, was in general more than 100 .mu.m, and a known method for producing such shadow masks was to carry out concurrent etching of a metal plate on the opposite sides thereof to produce through holes. This method is called a one-step etching method.
Another known method for producing a shadow mask was as follows. That is, a metal plate is applied with resin layers on the opposite front and rear surfaces thereof, and then pattern masks are applied to the opposite resin layers. A front pattern mask has one broad slit pattern and a back pattern mask has two adjacent narrow slit patterns. Subsequently, the front and rear pattern masks are printed to the front and rear resist layers, respectively, by exposure to light, and then developments on the resist layers of the printed front and rear patterns are made.
A half-etching is carried out on the rear surface of a metal plate through the developed rear resist layer to form adjacent narrow rear recesses in the rear surface of the plate. Then an etchant-proof resin is filled into the rear recesses and over the rear resin layer on the metal plate. Next a broad front recess is etched in the front surface of the metal plate through the developed front resist layer to cause the front recess to reach the half-etched rear recesses, whereby a through hole is produced in the metal plate. This method is a two-step etching method and is described in Japanese Patent Appln. Laid-Open (Kokai) No. 61-130,492 published Jun. 18, 1986.
Japanese Patent Application Laid-Open (Kokai) No. 5-28912 published Feb. 5, 1993 discloses a further method, using a front pattern mask having a broad slit pattern, and a rear pattern mask having two narrow slit patterns. This method aims at controlling the cross-sectional shapes of etched recesses and therefore the cross-sectional shape of the finally obtained through hole or slit.
Another method for producing a shadow mask is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 5-12,996 published Jan. 22, 1993. In this method, a metal plate is applied with resist layers on the front and rear surfaces thereof, and then a pattern slit mask is applied to only the front surface and printed to the same by exposure to light, while the rear resist layer is maintained as it is and backed up by a backup resin sheet. Etching is carried out on only the front surface of the metal plate through the printed and developed front resist layer to produce a front recess in the metal plate. The front recess reaches the rear resist layer, whereby a through hole is produced in the metal plate when the rear resist layer is removed together with the backup resin sheet.
The above stated known methods are for shadow masks of the conventional thickness of more than 100 .mu.m and cannot be used for thin shadow masks of a thickness of from 20 to 100 .mu.m for the following reasons.
Shadow masks are required to have not only dimensional precision but also precise shape control of the through holes to be formed. However, it is considered difficult to attain these requirements in the case of thin shadow masks. Shadow masks of any type, including aperture grills having vertical slits, used for CRT displays such as color televisions, have a function to control scanning of the electron beams within the cathode ray tube. Any type of shadow mask is a mask for selecting the electron beams, and the through holes or slits of the shadow masks are required to have a predetermined shape and taper of the internal side walls for allowing passage of the beams of selected angles. However, it has been difficult to produce through holes and slits having tapered side walls of strictly required tapering configuration.
More specifically, if the previously described one-step etching method wherein the front and rear side etchings are carried out concurrently is used for a thin metal plate of a thickness of from 20 to 100 .mu.m, the thin metal plate cannot withstand the spraying force within the etchant bath and tends to warp, losing its planar shape so that working precision drops.
In the case of use of the previously described two-step etching method for a thin metal plate of a thickness of from 20 to 100 .mu.m, the thin metal plate tends to be deformed during the filling of the resin so that the resin filling operation is time-consuming and troublesome.
When the one-side etching method disclosed in Japanese Patent Appln. Laid-Open (Kokai) No. 5-12,996 is used for a thin metal plate mentioned above, an etching quantity equivalent to that of a plate of double or triple thickness is required, and moreover it is difficult to produce a desired configuration of the through holes or slits with a resultantly obtained reduced degree of taper. If the etching quantity is reduced to secure a desired degree of taper, the degree of linearity of the side walls of the produced through holes or slits deteriorates so that dimensional precision of the holes or slits drops. Especially, in the case of production of aperture grills of a thickness of the order of 20 to 100 .mu.m, the slit shape becomes unstable at the instant of the breakthrough of the slit so that dimensional precision drops considerably and a desired degree of taper can hardly be obtained. When dimensional precision drops, there occurs non-uniform brightness when the aperture grill is seen through its slits and irregular depressions tend to be formed in the grill.